1. Field of the Invention
This invention relates generally to spin-electronic devices, more particularly to a magnetic tunnel junction and methods for making the same.
2. Description of the Related Art
Magnetoresistive elements having magnetic tunnel junctions (also called MTJs) have been used as magnetic sensing elements for years. In recent years, magnetic random access memories (hereinafter referred to as MRAMs) using the magnetoresistive effect of MTJ have been drawing increasing attention as the next-generation solid-state nonvolatile memories that can cope with high-speed reading and writing, large capacities, and low-power-consumption operations. A ferromagnetic tunnel junction has a three-layer stack structure formed by stacking a recording layer having a changeable magnetization direction, an insulating spacing layer, and a fixed layer that is located on the opposite side from the recording layer and maintains a predetermined magnetization direction.
As a write method to be used in such magnetoresistive elements, there has been suggested a write method (spin torque transfer switching technique) using spin momentum transfers. According to this method, the magnetization direction of a recording layer is reversed by applying a spin-polarized current to the magnetoresistive element. Furthermore, as the volume of the magnetic layer forming the recording layer is smaller, the injected spin-polarized current to write or switch can be also smaller. Accordingly, this method is expected to be a write method that can achieve both device miniaturization and lower currents.
Further, as in a so-called perpendicular MTJ element, both two magnetization films have easy axis of magnetization in a direction perpendicular to the film plane due to their strong magnetic crystalline anisotropy, shape anisotropies are not used, and accordingly, the device shape can be made smaller than that of an in-plane magnetization type. Also, variance in the easy axis of magnetization can be made smaller. Accordingly, by using a material having a large magnetic crystalline anisotropy, both miniaturization and lower currents can be expected to be achieved while a thermal disturbance resistance is maintained.
There has been a known technique for achieving a high MR ratio in a perpendicular magnetoresistive element by forming a crystallization acceleration film that accelerates crystallization and is in contact with an interfacial magnetic film having an amorphous structure. As the crystallization acceleration film is formed, crystallization is accelerated from the tunnel barrier layer side, and the interfaces with the tunnel barrier layer and the interfacial magnetic film are matched to each other. By using this technique, a high MR ratio can be achieved. However, where a MTJ is formed as a device of a perpendicular magnetization type, the materials of the recording layer typically used in an in-plane MTJ for both high MR and low damping constant as required by low write current application normally don't have enough magnetic crystalline anisotropy to achieve thermally stable perpendicular magnetization against its demagnetization field. In order to obtain perpendicular magnetization with enough thermal stability, the recording layer has to be ferromagnetic coupled to additional perpendicular magnetization layer, such as TbCoFe, or CoPt, or multilayer such as (Co/Pt)n, to obtain enough perpendicular anisotropy. Doing so, reduction in write current becomes difficult due to the fact that damping constant increases from the additional perpendicular magnetization layer and its associated seed layer for crystal matching and material diffusion during the heat treatment in the device manufacturing process.
In a spin-injection MRAM using a perpendicular magnetization film, a write current is proportional to the perpendicular anisotropy, the damping constant and inversely proportional to a spin polarization, and increases in proportional to a square of an area size. Therefore, reduction of an area size is mandatory technologies to reduce the write current.
Besides a write current, the stability of the magnetic orientation in a MRAM cell as another critical parameter has to be kept high enough for a good data retention, and is typically characterized by the so-called thermal factor which is proportional to the perpendicular anisotropy as well as the volume of the recording layer cell size. Although a high perpendicular anisotropy is preferred in term of a high thermal disturbance resistance, an increased write current is expected as a cost.
To record information or change resistance state, typically a recording current is provided by its CMOS transistor to flow in the stacked direction of the magnetoresistive element, which is hereinafter referred to as a “vertical spin-transfer method.” Generally, constant-voltage recording is performed when recording is performed in a memory device accompanied by a resistance change. In a STT-MRAM, the majority of the applied voltage is acting on a thin oxide layer (tunnel barrier layer) which is about 10 angstroms thick, and, if an excessive voltage is applied, the tunnel barrier breaks down. More, even when the tunnel barrier does not immediately break down, if recording operations are repeated, the element may still become nonfunctional such that the resistance value changes (decreases) and information readout errors increase, making the element un-recordable. Furthermore, recording is not performed unless a sufficient voltage or sufficient spin current is applied. Accordingly, problems with insufficient recording arise before possible tunnel barrier breaks down.
In the mean time, since the switching current requirements reduce with decreasing MTJ element dimensions, STT-MRAM has the potential to scale nicely at even the most advanced technology nodes. However, making of small MTJ element leads to increasing variability in MTJ resistance and sustaining relatively high switching current or recording voltage variation in a STT-MRAM.
Reading STT MRAM involves applying a voltage to the MTJ stack to discover whether the MTJ element states at high resistance or low. However, a relatively high voltage needs to be applied to the MTJ to correctly determine whether its resistance is high or low, and the current passed at this voltage leaves little difference between the read-voltage and the write-voltage. Any fluctuation in the electrical characteristics of individual MTJs at advanced technology nodes could cause what was intended as a read-current, to have the effect of a write-current, thus reversing the direction of magnetization of the recording layer in MTJ. Majorities of cell-to-cell variations come from the MTJ cell patterning process.
The MTJ patterning process becomes one of the most challenging aspects of manufacturing. Conventional techniques utilized to pattern small dimensions in a chip, such as ion milling etching (IBE) or reactive ion etching (RIE), having been less than satisfactory when applied to magnetic tunnel junction stacks used for MRAM. In most cases when these techniques are used, it is very difficult or almost impossible to cleanly remove etched materials without partial damages to magnetic tunnel junction properties and electric current shunting. In a RIE etching of magnetic material, physical sputtering is still the major component which unavoidable results in the formation of re-deposited residues that can short circuit the junctions of the MTJ or create shunting channel of the MTJ, yielding high resistance variations and serious reliability issues.
Another problem of conventional patterning techniques is the degradation of the recording layer and reference layer in the MTJ, due to corrosion caused by chemical residue remaining after etching. Exposure to reactive gases during refilling deposition of dielectrics such as silicon dioxide or silicon nitride after the MTJ etching can also cause corrosion. After refilling of dielectric material, a chemical mechanic polishing process is required to smooth out the top surface for bit line fabrication, which introduces a big manufacturing challenging as well as high cost and further corrosion.
The current fabrication method to form STT-MRAM is by etching and dielectric refilling. Due to the non-volatile nature of the etched magnetic materials, the sensor profile is typically sloped with small top (230) and large bottom (210, FIG. 2, prior art). As the result, the formed sensor size cannot be made small enough to reduce the current for information writing. Often the etched sensor edge got damaged with electrical shorting across the MgO barrier (220).
Thus, it is desirable to provide a greatly improved method or innovative method that enables well-controllable and low cost fabrication in MTJ patterning while eliminating damage, degradation and corrosion.